The present invention generally relates to a general switch and a switching method, and more particularly to a general switch which includes a word oriented switch and a bit oriented switch.
Public transmission networks for telecommunications generally operate at a bit transfer rate of 64 kbit/s, transmitting digital signals on transmission links that are adapted for this bit rate. The signal data normally appear in time slots, and in conventional networks, each time slot is reserved for a specific channel.
However, many of the operator exchangers connected to the public transmission network only require bit transfer rates that are much lower than 64 kbit/s. Examples of such lower bit transfer rates, so called subrates, are 8 or 16 kbit/s. Referring to FIG. 1, in digital mobile telephony for example, speech information is normally coded by a speech coder in such a way that a lower bit transfer rate over the radio link between the mobile unit MOBILE and the base station BS is obtained. In the GSM system, for instance, traffic between a mobile unit MOBILE and a base station BS is generally transmitted at a relatively low bit transfer rate, normally less than 16 kbit/s (GSM fullrate 13.0 kbit/s, GSM halfrate 5.6 kbit/s). In general, several base stations BS are connected to a base station controller BSC which handles the traffic between the base stations BS and the corresponding mobile units MOBILE. The traffic between the base station controllers BSC and mobile switching centers MSC often utilizes the normal public transmission network which operates at the higher bit transfer rate of 64 kbit/s. If the base station controllers BSC would switch GSM traffic of a bit rate less than 16 kbit/s directly onto a 64 kbit/s transmission network, a lot of bandwidth capacity would be wasted. Each channel would then occupy only one fourth or one eight of the bit positions of its reserved time slot. However, by circuit switching this traffic on bit level instead of word level, it is possible to fully utilize the bandwidth capacity offered by the normal public transmission network by packing several subrate channels into a new and fully packed time slot that is subsequently sent over the public transmission network.
According to the prior art, bit level circuit switching is normally provided by connecting an external bit oriented switch, also referred to as a subrate switch, in series with the ordinary word oriented switch with a connection from the bit oriented switch back to the word oriented switch. FIG. 2 is a schematic diagram illustrating such a prior art arrangement of a word oriented switch WSW and a bit oriented switch BSW. The word oriented switch WSW and the bit oriented switch BSW have input terminals for receiving data and output terminals for outputting data. The word oriented switch WSW is characterized in that only entire words can be processed.
Now, assume that the word oriented switch WSW receives subrate traffic at a first input terminal IN1. The subrate traffic appears in time slots, in which only a portion of the bit positions is occupied. The subrate traffic is switched through the word oriented switch WSW on word level, and outputted at a first output terminal OUT1. The first output terminal OUT1 of the word oriented switch WSW is connected to an input terminal IN of the bit oriented switch BSW. In the bit oriented switch BSW bit level switching is performed to form new words in which a number of subrate traffic channels are packed. The normal procedure is to form words that are fully packed. These new words are outputted from the bit oriented switch BSW at an output terminal OUT. The output terminal OUT of the bit oriented switch BSW is connected to a second input terminal IN2 of the word oriented switch WSW, and the received words are switched through the word oriented switch SWS on word level and outputted from a second output terminal OUT2 to the public transmission network over terminal circuits or equivalents.
Although the above prior art arrangement makes it possible to improve the bandwidth capacity utilization of the public transmission network, it still has a number of drawbacks. First of all, two different switches have to be operated to set up subrate connections. Second, the subrate traffic first has to be switched through the ordinary word oriented switch, through the bit oriented switch and then back to the word oriented switch, and once again switched through the word oriented switch. This naturally leads to substantial delays of the traffic. In addition, the input and output terminals of the ordinary switch that are connected to the bit oriented switch can not be used for normal rate traffic.
U.S. Pat. No. 5,453,985 discloses a general digital switch for a PCM transmission system for switching PCM words on both word and bit level. The general switch comprises a word oriented switch and a bit oriented switch, the bit oriented switch being connected in series with the word oriented switch with a connection from the bit oriented switch back to the word oriented switch.
U.S. Pat. No. 4,718,058 discloses a complicated multi-stage switching network with a plurality of switching stages connected in series with each other. In the multi-stage switching network the input and output stages are arranged to switch on bit level.
The present invention overcomes these and other drawbacks of the prior art arrangements.
It is a general object of the present invention to provide a general switch which supports word oriented switching as well as bit oriented subrate switching. Furthermore, it is advantageous if the set up of subrate connections through the general switch is simple, and the traffic delay for subrate traffic is reduced to a minimum.
Another object of the invention is to provide a general switch which supports normal rate and subrate switching, and which is scaleable with regard to switching capacity. In particular, it is desirable to provide such a general switch in modular form since modular arrangements have advantages with regard to costs and flexibility. In addition, relatively high subrate switching capacities such as 32K or more should be easy to realize with such a modular switch.
Still another object of the invention is to provide a switching method in a general switch which supports normal rate and subrate switching, and according to which the set up of subrate connections is simple, and the traffic delay for subrate traffic is minimized.
These and other objects are met by the invention.
In accordance with a general inventive concept there is provided a general switch in which a word oriented switch and a bit oriented switch are connected such that a common input signal is provided to both of the switches. The common input signal comprises data appearing in time slots. The general switch further comprises means for establishing, for each one of a number of outgoing time slots used for traffic, at least one of a first connection path through the word oriented switch and a second connection through the bit oriented switch, and means for selecting, for each outgoing time slot, data from an established one of the connection paths.
In a preferred embodiment of the invention, for each outgoing time slot, the establishing means establishes only one of the first connection path and the second connection path and the selecting means selects data from the established connection path.
In this way, a dual mode general switch is provided, in which word oriented switching is performed in a first operation mode and bit oriented switching is performed in a second operation mode.
A major advantage of the solution according to the invention is that the traffic delay for subrate traffic is substantially reduced compared to prior art arrangements.
To set up a subrate connection through the general switch it is consequently sufficient to establish a connection path only through the bit oriented switch and not through the word oriented switch. In this way the set up of subrate connections is also simplified.
Other advantages offered by the present invention will be appreciated upon reading of the below description of the embodiments of the invention.